1. Field of the Invention
This invention relates to concrete extrusion machines for forming extrusions of concrete articles, such as floor slabs, panels and beams, and in particular to the production of cored slabs of concrete.
2. Prior Art
Conventionally, in extrusion machines for making cored concrete slabs, a travelling mold into which concrete is fed by a plurality of augers to which forming mandrels and finishing tubes are connected in a string, may use a vibrating plate spanning a moulding box which compacts the concrete slab as it is being formed and, due to vibrations imparted to the concrete, ease the passage of the mandrels and forming tubes through the concrete mass. Further, most such machines use a finishing plate following the vibrating plate which smoothes the top surface of the concrete and also use vibrators internally of the mandrel to further port the passage of the mandrel and finishing tubes through the concrete mass.
It has been found that, due to transmission of vibrations through the concrete mass, some settlement over the hollow cores takes place as the slab passes beneath the finishing plate so that the finished slab present a wavy upper surface.
Due to such settlement, the density of the concrete above each core is thus reduced. Also due to the friction of the finishing plate which retards movement of the extrusion machine when the slab is being formed, a certain amount of concrete will back-feed by the augers.